DESCRIPTION: The overall goal of the research is to identify and characterize mutants and genes affecting structure and function in the nucleus, using S. cerevisiae as a model organism. There are three specific aims. The first is to use the two hybrid system to identify proteins that interact with DNA topoisomerases I and II and to characterize the in vivo roles of these proteins through standard genetic methods. The second is to continue studies on transcriptional silencing in yeast. SIR1, SIR3, and SIR4 homologs will be isolated from related yeasts, and, if possible, from S. pombe and metazoans to determine whether silencing by Sir proteins is evolutionarily conserved. The domains required for Sir1 functions in silencing will be identified. The role of S phase in the establishment of silencing by Sir1 will be determined, using as an assay Sir1 tethered to a silencer from which the ARS element has been deleted. Two-hybrid screens will be used with SIR1 and the N-terminus of SIR3 to identify interacting proteins. The third specific aim is to find mutants defective in the maintenance of nuclear integrity using a the fluorescent dye DiOC6 and to isolate the corresponding genes. Nuclear lamin genes in budding and fission yeast will be identified by screening for mutants that depend on the expression of the chicken lamin B gene for viability. A similar genetic approach will be applied to SIR3 and SIR4 to identify functionally equivalent genes that may participate in the attachment of telomeres to the nuclear periphery.